Over the last year or two remarkable strides have been made in the field of superconducting materials. Previously it was thought to be superconducting, materials (which predominantly were metallic) had to be kept at temperatures on the order of 15.degree.-20.degree. K. or lower. However, a series of recent discoveries in the field of ceramics has led to the appearance of a variety of materials which are superconducting at temperatures of 100.degree. K. or greater. These, of course, offer great potential since their superconducting properties can be utilized at temperature conditions which are easily and routinely maintained with liquid nitrogen cryogenic equipment, rather than being limited to use only in the exacting and difficult to obtain liquid helium temperature near absolute zero.
A key property of superconductivity is the ability to maintain a high current density in the superconducting material. The particular current density involved will be dependent upon whether the superconducting material is in the form of single crystals and thin films or in bulk and polycrystalline form; Cava et al., Phys. Rev. Ltrs., 58, 16, 1676 (1987). It has been stated that current density can be increased by ordering the grains in bulk superconducting ceramics; Anon., Superconductor Week, 2, 1, 3 (Jan. 4, 1988). Grain ordering has been accomplished to some degree by melting the superconducting material and then cooling it to achieve a more aligned grain orientation; Jin et al., reported in High Tc Update, 1, 16, 1 (Dec. 15, 1987). Another approach taken by researchers has been to apply a magnetic field to a bulk superconducting ceramic after the material has been completely formed; Finnemore, reported in Sci. Amer., 257, 4, 32 (Oct. 1987). Neither of these techniques has been particularly successful, and current densities high enough for commercial applications have not been achieved to date. In addition, melting of the material's crystalline structure imparts significant changes which can adversely affect not only superconductivity but also other properties of the material. Further, the post-formation magnetic field application achieves only a very limited success since the grain orientations are essentially fixed upon cooling and are only reoriented incompletely and with considerable difficulty.
Many of the new ceramic materials are classified as cuprates. A number of articles describing the formulas, structures, properties and preparation of cuprates have recently appeared: notable among these are Hazen, Sci. Amer., 258, 6, 74 (June 1988); Thompson et al, Phys. Rev. B, 36, 1, 836 (1987); Chen et al., Rev. Sci. Instrum., 58, 9, 1565 (September 1987) and Haldar et al., Science, 241. 1198 (Sept. 2, 1988).
Various studies have shown that the superconductivity of the metal cuprates is a function of the oxygen content of the repeating crystalline unit; Hazen, supra; Tarascon et al., Phys. Rev. B, 36, 1, 226 (1987). It has been found that the superconductivity is dependent upon elemental oxygen vacancies in the unit lattice, with both the number of vacancies and their locations being significant; Anon., Metal Progress. p. 76 (August 1987) and Schwartz, Adv. Cer. Matls., 2, 4, 753 (1987). Thus much of the reported research has been directed toward controlling the oxygen content of the superconducting cuprates.